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© 2015 AOAC INTERNATIONAL
AOAC Official Method 2015.01
Heavy Metals in Food
Inductively Coupled Plasma–Mass Spectrometry
First Action 2015
Note : The following is not intended to be used as a comprehensive
training manual. Analytical procedures are written based on the
assumption that they will be performed by technicians who are
formally trained in at least the basic principles of chemical analysis
and in the use of the subject technology.
{Applicable for the determination of heavy metals [arsenic
(As), CAS No. 7440-38-2; cadmium (Cd), CAS No. 7440-43-
9; lead (Pb), CAS No. 7439-92-1; and mercury (Hg), CAS No.
7439-97-6] at trace levels in food and beverage samples, including
solid chocolate, fruit juice, fish, infant formula, and rice, using
microwave digestion and inductively coupled plasma–mass
spectrometry (ICP-MS).}
Caution : Nitric acid and hydrochloric acid are corrosive. When
working with these acids, wear adequate protective gear,
including eye protection, gloves with the appropriate
resistance, and a laboratory coat. Use an adequate fume
hood for all acids.
Hydrogen peroxide is a strong oxidizer and can react
violently with organic material to give off oxygen gas
and heat. Adequate protective gear should be worn.
Many of the chemicals have toxicities that are not well
established and must be handled with care. For all known
chemicals used, consult the Material Safety Data Sheet
(MSDS) in advance.
The inductively coupled plasma–mass spectrometer
emits UV light when the plasma is on. UV resistant
goggles should be worn if working near the plasma.
The instrument generates high levels of radio frequency
(RF) energy and is very hot when the plasma is on. In the
case of an instrument failure, be aware of these potential
dangers.
Safely store interference reduction technology (IRT)
gases, such as oxygen, in a closed, ventilated cabinet. Use
adequate caution with pressurized gases. Prior training
or experience is necessary to change any gas cylinders.
Oxygen gas can cause many materials to ignite easily.
Following microwave digestion, samples are hot to the
touch. Allow the samples to cool to room temperature
before opening the digestion vessels to avoid unexpected
depressurization and potential release of toxic fumes.
A. Principle
Food samples are thoroughly homogenized and then prepared
by microwave digestion and the addition of dilute solutions of
gold (Au) and lutetium (Lu). The Au is used to stabilize the Hg in
the preparation, and the Lu is used to assess the potential loss of
analyte during the microwave digestion process.
Aprepared, diluted, aqueous sample digestate is pumped through
a nebulizer, where the liquid forms an aerosol as it enters a spray
chamber. The aerosol separates into a fine aerosol mist and larger
aerosol droplets. The larger droplets exit the spray chamber while
the fine mist is transported into the ICP torch.
Inside the ICP torch, the aerosol mist is transported into a high-
temperature plasma, where it becomes atomized and ionized as it
passes through an RF load coil. The ion stream is then focused
by a single ion lens through a cylinder with a carefully controlled
electrical field. For instruments equipped with dynamic reaction cell
(DRC) or collision cell IRT, the focused ion stream is directed into
the reaction/collision cell where, when operating with a pressurized
cell, the ion beam will undergo chemical modifications and/or
collisions to reduce elemental interferences. When not operating
with a pressurized cell, the ion stream will remain focused as it
passes through the cell with no chemical modification taking place.
The ion stream is then transported to the quadrupole mass
filter, where only ions having a desired mass-to-charge ratio (m/z)
are passed through at any moment in time. The ions exiting the
mass filter are detected by a solid-state detector and the signal is
processed by the data handling system.
B. Equipment
Perform routine preventative maintenance for the equipment
used in this procedure.
An ultra-clean laboratory environment is critical for the
successful production of quality data at ultra-low levels. All sample
preparation must take place in a clean hood (Class 100). Metallic
materials should be kept to a minimum in the laboratory and coated
with an acrylic polymer gel where possible. Adhesive floor mats
should be used at entrances to the laboratory and changed regularly
to prevent the introduction of dust and dirt from the outside
environment. Wear clean-room gloves and change whenever
contact is made with anything non-ultra-clean. The laboratory floor
should be wiped regularly to remove any particles without stirring
up dust. Note: “Ultra-clean” (tested to be low in the analytes of
interest) reagents, laboratory supplies, facilities, and sample
handling techniques are required to minimize contamination in
order to achieve the trace-level detection limits described herein.
(a ) Instrumentation.—ICP-MS instrument, equipped with IRT
with a free-running 40 MHz RF generator; and controllers for
nebulizer, plasma, auxiliary, and reaction/collision flow control.
The quadrupole mass spectrometer has a mass range of 5 to 270
atomic mass units (amu). The turbo molecular vacuum system
achieves 10–6 torr or better. Recommended ICP-MS components
include an RF coil, platinum skimmer and sampler cones, Peltier-
cooled quartz cyclonic spray chamber, quartz or sapphire injector,
micronebulizer, variable speed peristaltic pump, and various types of
tubing (for gases, waste, and peristaltic pump). Note: The procedure
is written specifically for use with a PerkinElmer ELAN DRC II
ICP-MS . Equivalent procedures may be
performed on any type of ICP-MS instrument with equivalent IRT
if the analyst is fully trained in the interpretation of spectral and
matrix interferences and procedures for their correction, including
the optimization of IRT. For example, collision cell IRT can be used
for arsenic determination using helium gas.
(b ) Gases.—High-purity grade liquid argon (>99.996%).
Additional gases are required for IRT (such as ultra-x grade,
99.9999% minimum purity oxygen, used for determination of As
in DRC mode with some PerkinElmer ICP-MS instruments).
(c) Analytical balance.—Standard laboratory balance suitable
for sample preparation and capable of measuring to 0.1 mg.
(d ) Clean-room gloves.—Tested and certified to be low in the
metals of interest.
Candidates for 2016 Method of the Year
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